Thirteen Corn Belt senators, led by Minnesota Democrat Amy Klobuchar and Iowa Republican Chuck Grassley, have petitioned the U.S. Environmental Protection Agency to stop issuing waivers that exempt certain oil refineries from federal biofuel laws.

A U.S. District Court judge in Minnesota has shot down an attempt by trucking, automotive and oil industry groups to abolish the state’s requirement that diesel sold at the pump contain at least 10% biofuel.